Transitional Kindergarten (TK) is a relatively new model of early childhood education, with little evidence on whether and how it affects children's development. This study provides new evidence using data from Michigan, which has the nation's second-largest TK program. Using survey data (N=171) from administrators in 2021-2022, the paper documents several program features that distinguish TK from typical public Pre-K programs (including Michigan's), such as greater use of domain-specific curriculum. Focusing on students who enrolled at four years old (50% female, 78% White), a regression discontinuity analysis shows that Michigan TK improved children's readiness for kindergarten (0.91 SD, N=1,943), improved third-grade math scores (0.2-0.3 SD, N=15,680), and may have caused earlier entry into special education followed by earlier exit (N=15,704).